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    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
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    <script>
        function queryData(url) {
            return new Promise(function (resolve, reject) {
                var xhr = new XMLHttpRequest();
                xhr.onreadystatechange = function () {
                    if (xhr.readyState != 4) return;
                    if (xhr.readyState === 4 && xhr.status == 200) {
                        //处理正常的情况
                        resolve(xhr.responseText);
                    } else {
                        //处理异常情况
                        reject('服务器错误')
                    }
                };
                xhr.open('get', url);
                xhr.send(null);
            });
        };

        //发送多个ajax请求并且保证顺序
        queryData('http://localhost:3000/data')
            .then(function (data) {
                console.log(data);
                return queryData('http://localhost:3000/data1') //返回promise对象
            })
            .then(function (data) {
                console.log(data);
                return new Promise(function (resolve, reject) { //返回promise对象
                    setTimeout(function () {
                        resolve(123);
                    }, 1000)
                });
            })
            .then(function (data) {
                console.log(data);
                return 'hello' //返回普通值,会默认产生一个新的promise对象让下一个then调用
            })
            .then(function (data) {
                console.log(data);
            })
    </script>
</body>

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